Eyetracking-enhanced VEP for nystagmus

Visual evoked potentials (VEPs) are an important prognostic indicator of visual ability in patients with nystagmus. However, VEP testing requires stable fixation, which is impossible with nystagmus. Fixation instability reduces VEP amplitude, and VEP reliability is therefore low in this important patient group. We investigated whether VEP amplitude can be increased using an eye tracker by triggering acquisition only during slow periods of the waveform. Data were collected from 10 individuals with early-onset nystagmus. VEP was obtained under continuous (standard) acquisition, or triggered during periods of low eye velocity, as detected by an eye tracker. VEP amplitude was compared using Bonferroni corrected paired samples t-tests. VEP amplitude is significantly increased when triggered during low eye velocity (95% CI 1.42–6.83 µV, t(15) = 3.25, p = 0.0053). This study provides proof-of-concept that VEP amplitude (and therefore prognostic reliability) can be increased in patients with early onset nystagmus by connecting an eye tracker and triggering acquisition during periods of lower eye velocity.

Velocity-triggering resulted in a mean of 121 VEP epochs per recording (± 25 standard error), whereas continuous acquisition resulted in 134 epochs per recording (± 12 standard error).A paired-samples t-test found no significant difference in the number of epochs obtained between acquisition methods (95% CI − 67 to 78, t(10) = 0.16, p = 0.87).
A linear regression model was fitted to examine the relationship between trigger velocity threshold and the proportional increase in VEP amplitude using velocity triggering.This was not significant (p = 0.16), indicating that improvements in VEP amplitude do not depend on the velocity trigger threshold used across participants.Figure 3 shows an example averaged VEP waveform and a portion of the eye speed trace which provided the triggers for VEP acquisition.

Discussion
This study demonstrates that VEP amplitude can be increased in early onset nystagmus by triggering VEPs when the eyes move slowly.This method has the potential to improve prognostic reliability.
Previous studies investigated periods of low eye velocity during continuous VEP acquisition 5,7 , whereas our method triggers VEP acquisition only when the eyes are moving slowly.We suggest two possible reasons that  VEP amplitude is higher when the eyes move slowly.One is that reduced motion blur when the eyes are slowed improves sensitivity, but this is at odds with evidence that motion blur does not limit visual acuity in adults with IN 8 (only one participant in the present study was within the critical period of visual development).Another possibility is that visual sensitivity is reduced during non-foveating periods of the nystagmus waveform, but the 'visual sampling' hypothesis is also contested 9 .
IN waveforms typically include regular foveation periods during which eye velocity is slowest.The 'velocity trigger' used in our method is therefore likely best suited to IN waveforms with established foveations periods.Four of our participants with IN (P02, P03, P07 and P10) exhibited well-defined foveation periods; further research with a larger sample size could determine the extent to which a patient's foveation quality predicts the benefits of using our method.Further investigation in patients with late-onset nystagmus may also be warranted.
In conclusion, we provide proof-of-concept that triggering VEP acquisition during slower portions of the nystagmus waveform increases signal amplitude (and therefore reliability) in patients with early onset nystagmus.

Methods
The study was conducted at the Eye Unit's Electrodiagnostic Department at University Hospital Southampton.The data collected in the study were approved by the University Hospital Southampton Foundation Trust Governance Committee.The investigation was carried out in accordance with the Declaration of Helsinki; informed consent was obtained from the participants after explanation of the nature and possible consequences of the study.Study participants were identified through a regional nystagmus clinic.All participants had a full history taken and underwent orthoptic assessment, ophthalmic examination of anterior and posterior segments using age-appropriate equipment, detailed nystagmus examination, and optical coherence tomography (OCT) of their maculae using age-appropriate equipment, according to a standard nystagmus evaluation workflow 4,10 .Ten participants with nystagmus took part in the study; Table 1 provides clinical details, including nystagmus subtype.For the purposes of analysis, participants whose nystagmus developed in infancy were classified as 'early onset' .
Clinical visual acuity was measured either by the orthoptist or referring clinician and converted to logMAR where appropriate.Participants took as long as they wished to view the chart and were encouraged to use their null zone, if present.VEPs were then acquired using an Espion 300 Desktop System (Diagnosis LLC, Cambridge, UK) running Espion software version 6.64.14, and VEPs were recorded following ISCEV guidelines for pattern reversal and onset-offset VEPs.Recordings were made with silver/silver chloride electrodes adhered to the skin using 10/20 conductive paste after preparation with SkinPure gel (Nihon Kohden, Tokyo, Japan) to ensure impedances were equal and < 5 kΩ.Electrode placement followed the Queen Square protocol with active electrodes at O1, Oz, O2, and reference and earth placed on the forehead.Recording settings were as follows: 1000 Hz sample rate, 1 Hz low frequency filter and 100 Hz high frequency filter.Room lights were off.VEP acquisition was triggered externally via a LabJack U6 (LabJack, Lakewood, CO, USA) under two conditions ('continuous' and 'velocity triggered').For each condition, both onset-offset and pattern reversal VEPs were acquired (six participants undertook pattern reversal VEPs, and all 10 participants undertook onset-offset VEPs) using a 50′ chequerboard pattern at a viewing distance of 1 m on a 602 × 341 mm monitor (GEST270HB-OD01, ESTECOM, Gunpo, South Korea) and 1600 × 900 pixel resolution, 60 Hz refresh rate and 200 cd/m 2 maximum luminance.The head was stabilized by a chin rest.Horizontal and vertical eye position were monitored at 500 Hz by an EyeLink 1000 video-based eye tracker (SR Research, Ottawa, ON, Canada; firmware version 5.15) using a Desktop Mount.The left eye was tracked unless there was a marked difference in interocular visual acuity, in which case the eye with the better vision was tracked.Eye tracker calibration, data processing and segmentation of nystagmus waveforms (as reported in Table 1), were achieved using the method described by Dunn et al. 11 .Stimulus presentation was controlled using MATLAB (The MathWorks, Inc., Natick, MA), using Psychophysics Toolbox extensions [12][13][14] .A 0.25° red circular fixation target was presented in the straight-ahead position.For onset-offset VEPs, the pattern was shown for 250 ms and disappeared for a minimum duration of 295 ms; for pattern reversal VEPs, reversals occurred at minimum intervals of 333 ms.In the 'continuous' condition, VEPs were acquired back-to-back at regular intervals (according to the minimum durations above).In the 'velocity triggered' condition, VEP acquisition (either an onset-offset pair or pattern reversal, as appropriate) took place whenever eye velocity fell below a threshold, determined per-participant (see below).Eye velocity was calculated live by differentiation of gaze position over the preceding 10 ms, using a 5 ms averaging window.VEP acquisition was triggered when eye velocity exceeded threshold, with acquisition frequency limited to the minimum intervals specified above.Since nystagmus intensity varies across individuals (and in some cases within an individual), VEP trigger threshold was empirically determined at the start of each recording by adjustment by the experimenter until high enough to trigger, but not so high as to trigger continuously (thresholds ranged from 0.5 to 26°/s [mean 7.8 ± 1.5°/s standard error]).Using a photodiode, the total latency of the video hardware was measured as ~ 24 ms; the total latency between the eye velocity trigger and VEP acquisition was adjusted to also be ~ 24 ms (ensuring that stimulus onset coincided with VEP acquisition, both of which occurred ~ 24 ms after the appropriate eye velocity was detected).Using the R Environment for Statistical Computing 15 , a paired-samples t-test was conducted to compare VEP amplitude under each trigger condition (velocity triggered vs continuous).

Figure 1 .
Figure 1.Amplitudes obtained for onset-offset VEPs using both standard and velocity-based triggering.

Figure 2 .Figure 3 .
Figure 2. Amplitudes obtained for pattern reversal VEPs using both standard and velocity-based triggering.Note that P02 and P04 are coincident.